1. Field of the Invention
This invention relates to an electrically rewritable nonvolatile memory and a data read method and more particularly to the technique for compensating for temperature dependency of a memory cell current.
2. Description of the Related Art
With increasing miniaturization of semiconductor integrated circuit devices, the width and thickness of wirings are becoming smaller and the wiring resistance becoming higher. For example, in a semiconductor memory device, with increasing miniaturization of the wiring, the resistance of bit lines is made higher. Generally, a sense amplifier for sensing and amplifying data read from a memory cell is connected to the bit lines. In the data read operation, a memory cell current is set to “Cb1×ΔVb1/tb1” in a portion near the sense amplifier (that is, in a portion in which the resistance of the bit line is low) if it is required to change the potential of the bit line by ΔVb1 in a preset read period tb1 and the bit line capacitance is set to Cb1. On the other hand, since the resistance of the bit line becomes higher in the end portion of the bit line which is farthest apart from the sense amplifier, a larger memory cell current is required.
Therefore, the memory cell current varies according to the distance of a portion of the bit line from the sense amplifier (the distance from a sense point). If the memory cell current varies according to the distance from the sense point, the temperature characteristic of the memory cell is changed and the threshold voltage thereof fluctuates according to temperature variation. If the threshold voltage fluctuates, the distribution of the threshold voltages spreads and the data holding characteristic thereof is degraded (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2003-217287 and Jpn. Pat. Appln. KOKAI Publication No. 2000-011671).
Further, the semiconductor memory device will be further miniaturized in future, and it is predicted that the temperature characteristic of the memory cell varies and the threshold voltage fluctuates according to the difference in the etching rate due to the difference in the environment of the surroundings, aberration and distortion of the optical system of an exposure device caused by a difference in the pattern of peripheral portions such as regions in which the wirings are densely and coarsely arranged in a single chip or a difference in the position of a wafer, for example, the central portion or peripheral portion of the wafer in which chips of the semiconductor memory devices are formed.
Therefore, there occurs a possibility that the distribution of the threshold voltages will spread due to a difference in the position of the memory cells in the chip or a difference between the chips and the data holding characteristic is deteriorated.